where term is a term that when evaluated will produce a theory
(see theories), and doc-string is an optional documentation
string not beginning with ``:doc-section ...''. Except for the
variable world, term must contain no free variables. Term is
evaluated with the variable world bound to the current world to
obtain a theory and the corresponding runic theory
(see theories) is then made the current theory. Thus,
immediately after the in-theory, a rule is enabled iff its rule name
is a member of the runic interpretation (see theories) of some
member of the value of term. See theory-functions for a list
of the commonly used theory manipulation functions.